Recently, a secondary battery having a high capacity has been desired with miniaturization of electronic devices. Therefore, a lithium ion secondary battery has attracted attention, which has an energy density higher than a nickel-cadmium battery and a nickel-metal-hydride battery.
As its negative active material, it is first attempted to use metallic lithium but it is revealed that dendrite lithium deposits during repeated charge and discharge and passes through a separator to reach a positive electrode, whereby there is a possibility to induce a firing accident through short circuit. Therefore, at present, it attracts attention to use, as the negative active material, a carbon material capable of intercalation and deintercalation of lithium ions between layers and prevention of deposition of metallic lithium in the process of charge and discharge.
As the carbon material, for example, use of graphite is described in Patent Literature 1. In particular, when graphite having a large graphitization degree is used as the negative active material for a lithium secondary battery, a capacity close to 372 mAh/g that is a theoretical capacity of graphite for lithium intercalation is obtained and thus the above graphite is known to be preferred as the active material. However, there exists a problem that graphite has a large irreversible capacity due to co-intercalation into planes of graphite layers when an electrolyte containing propylene carbonate (PC) as a solvent is used.
Under such circumstances, Patent Literature 2 describes that the co-intercalation is reduced or prevented by employing, as the carbon material for the negative electrode of a lithium secondary battery, a carbon material obtainable by treating carbon fine particles with a silane coupling agent and subsequently oxidizing the particles to form a silica thin film on the surface. At formation of a negative electrode with a negative active material, a binder is necessary. One of the most popular binders is polyvinylidene difluoride (PVDF). Also in the cited literature 2, a slurry obtained by mixing the active material and PVDF is applied onto a collector to form the negative electrode.
PVDF has a repeating unit represented by the following formula. Thanks to a characteristic that fluorine atoms are present in the main chain structure, it has a good flexibility and a good dispersibility of an active material and is also apt to attach onto a particle surface. Therefore, PVDF is highly practical in view of the applicability but there exist problems that strength of the active material layer after application and drying tends to be insufficient owing to its flexibility as shown in Comparative Example 2 to be described below and swelling in an electrolyte is large.[—CH2—CF2—]
Furthermore, in recent years, as one approach to further improved high-performance battery, it is desired for the graphite material to be chargeable at a high current density but it is revealed that a conventional electrode having an active material layer composed of graphite and a binder is poor in this property. This is because the binder in the active material layer lowers a lithium-acceptability. Therefore, it may be considered to enhance the lithium-acceptability by reducing the binder content but this method weakens the strength of the active material layer at the same time. Particularly in a battery where the negative electrode is used by winding it together with a positive electrode and a separator, there arises a problem that the active material layer may exfoliate from the collector at an electrode winding process.    [Patent Literature 1]
JP-A-57-208079    [Patent Literature 2]
JP-A-11-329435